The myelodysplastic syndromes (MDS) are acquired clonal disorders of the bone marrow. They were clearly defined in morphological terms by the French-American-British (FAB) group in 1982, as five conditions each with their own diagnostic criteria, but with the shared characteristics of ineffective blood cell production in one or more cell line, morphological dysplasia and a variable propensity to evolve into acute myeloid leukaemia (AML). In clinical practice patients typically present in old age with macrocytic anaemia, cytopenias, monocytosis and accumulation of marrow blast cells leading in time to fatal bone marrow failure or AML. To date treatment is unable to alter the natural history of MDS except in those few individuals who are able to undergo allogeneic progenitor cell transplantation.